http://bugs.winehq.org/show_bug.cgi?id=21870
--- Comment #9 from edward savage epssyis@gmail.com 2010-04-21 04:03:58 --- I was reviewing bug 3254 and noticed the patch in post #77 to fix the issue outlined in that bug. While the patch does not fix this issue it does cause an alternative failure in steam which appears to be progress. Instead of a testshell error in steam and a blank white page there is a wine page fault with the address persisting in steam and a blank standard gray/black steam background. Before steam crashes as a result of the page fault it is possible to view source for where the checkout page would be (nothing is rendered). The checkout page is being generated in full (the purchase is listed, places to enter credit card details, etc) and sent to the client with the patch whereas without it you only get '<html><head></head><body></body></html>' and a white page.
It may be a good starting point to solve this problem?